1. Field of the Invention
This invention relates to methods for preventing the deposition of carbon, or coke, on fuel wetted surfaces located in high temperature zones of gas turbine engines. Coke deposition is an undesirable side effect caused by the catalytic-thermal degradation of hydrocarbon fuels during their consumption in gas turbine engines. Such deposition leads to performance loss, reduced heat transfer efficiencies, increased pressure drops, costly decoking procedures, and increased rates of material corrosion and erosion. The metals most prone to catalyze coke deposition are those metals commonly found in the alloys utilized in components exposed to high temperature, fuel wetted environments of gas turbine engines, typically found in jet engines in the combustor and afterburner fuel delivery systems.
2. Description of the Prior Art
Carburization, or the formation of coke deposits, has been noted particularly in high temperature environments where carbon containing fluids come in contact with metals or metal alloys. Exemplary of such environments are high temperature reactors, such as refinery crackers, thermal crackers, distillation units for petroleum feedstock, and gas turbine components. Conventional methods used to reduce coke formation and carburization in steam cracking operations involve the steam pretreatment of the surface to promote formation of a protective oxide skin. The surface may then be further protected by the deposition of a high temperature, stable, non-volatile metal oxide on the pre-oxidized substrate surface by thermal decomposition from the vapor phase of a volatile compound of the metal.
While the chemical vapor deposition of an alkoxysilane has been demonstrated to reduce the rate of coke formation in the pyrolysis section of an ethylene steam cracker by formation of an amorphous silica film on the internal surfaces of high alloy steel tubing at 700.degree. to 800.degree. C., no one to data has solved the problem of coke deposition on fuel contacting hardware in gas turbine engines.
Alumina coatings have been applied to a large number of substrates for various purposes, but never, to our knowledge, for the prevention of coke deposition on fuel contacting elements in gas turbines. For example, flame sprayed coatings of alumina have been applied to foundry molds, but lacked adherence due to thermal shock. In U.S. Pat. No. 2,903,375, Peras attempted to overcome this problem by applying layered coatings of cermets containing alumina and chromium. Montgomery et al, in U.S. Pat. No. 2,775,531, suggest the application of aluminum-alumina cermets to metal substrates by flame spraying and sintering to provide high temperature oxidation resistance and thermal insulation. In U.S. Pat. No. 3,839,618, Muehlberger teaches spray coating stainless steel with a dielectric layer of alumina. Hecht, in U.S. Pat. No. 4,034,142, teaches the protection of nickel and cobalt superalloy articles at elevated temperatures by the formation of a coating having an external continuous layer composed predominately of alumina, which reduces oxidation and corrosion.